1. Field of the Invention
This invention relates generally to systems directed toward monitoring welding processes, and more particularly to systems which are adapted to provide immediate, real-time indications of degradation in a weld bead from an optimum weld bead as the acutal weld bead is bing produced.
2.Related Art
There are two commonly used field welding processes-- shielded metal-arc (stick electrodes) and gas metal-arc (bare wire). In both types, the source of heat for melting the weld material is an electric welding arc. Such an electric welding arc can be viewed as a gaseous conductor that changes electrical energy into heat. The welding arc can be defined as a particular group of electrical discharges that are formed and sustained by the development of a gaseous conductive media. The current carriers for the gaseous medium are produced by thermal means and field emission.
The arc current is carried by the plasma, the ionized state of a gas, which is composed of nearly equal numbers of electrons and ions. Mixed with the plasma are other states of matter, such as molten metals, slags, vapors, neutral and excited gaseous atoms and molecules.
During the welding process, changes in arc voltage, travel speed, and heat input can occur without the operator's knowledge. These changes can cause defects in the deposited metal weld bead.
Porosity is a weld defect consisting of a void or gas pocket trapped in the solidifying weld metal. The reduced solubility of the gas in the metal caused by the decreasing temperature forces the gas out of molten solution. Gases are originally introduced either by poor shielding, which contains air, or by chemical reactions in the molten metal weld. With stick electrodes, too long an arc resulting from excessive arc voltage can reduce the shielding effectiveness, thus introducing gas.
Slag inclusion is another defect caused by the entrapment of an oxide or other non-metallic material under the weld seam. The major source of slag is the coating on stick electrodes. This defect is also related to the heat input to the weld.
Incomplete fusion is another defect caused by the failure of adjacent layers of the weld metal or weld base plate to fuse. Incomplete fusion may result when the adjacent metal is not heated to the melting point because of insufficient heat input.
Another defect is an undercut, which is a groove melted into the base plate at the toe of the weld and is caused primarily by excessive travel speed in relation to the welding current.
The cost of locating and repairing the defects set forth above can be a major portion of weld construction costs. Welding inspection, for example, can constituted 25%-40% of the weld fabrication costs. In addition, weld defects decrease service life of welded joints.
The cooling cycle after a weld pass determines the microstructure of the weld metal in the heat-affected zone. With fast cooling rates, some steels become very hard because of martensitic transformation. If the cooling is sufficiently slow, the metal may become more ductile, and the structure ferritic and pearlitic. The type of steel generally determines which of these structures is desired. For low carbon and low alloy steels, the pearlitic structure is desirable, while, for high strength quenched and tempered steel, the martensitic structure is desirable.
Control of the arc parameters can be used to control the amount of heat generated in the welding process, the length of time the weld material remains at an elevated temperature and the cooling rate of the weld zone. Consequently, it is desirable to monitor the appropriate welding parameters in order to detect, identify and locate variations from optimum parameters. A weld monitor with a real-time output would aid the inspector in designating suspect areas for non-destructive testing after welding. Further, a real-time weld quality monitor could be used to interpret welding when defects are occurring, thus precluding costly rework.
Various attempts have been made at providing systems for monitoring certain weld parameters. For example, U.S. Pat. No. 3,236,997 to Johnson et al shows a system for controlling the length of a welding arc of a consumable electrode welding system. The control system employs a photosensitive transducer, such as a photodiode, which is positioned to sense the illumination emitted by the arc.
U.S. Pat. No. 3,262,006 to Sciaky et al shows a welding control system which is responsive to the radiation intensity of the welding arc for controlling the position of a nonconsumable electrode.
U.S. Pat. No. 3,370,151 to Normando shows a control system using radiant energy detector scanning. The system employs a radiation sensitive means, such as a photocell, which is sensitive to infrared radiation or, alternatively, to a mixture of infrared and visible red radiation.